1. Field of the Invention
The present invention relates to a single cell for a solid oxide fuel cell operating at low temperatures, more particularly to a single cell for a solid oxide fuel cell operating at low temperatures which is suitable for a distributed power source, a cogeneration system or the like in urban areas.
2. Description of Related Art
A solid oxide fuel cell (hereinafter referred to as an “SOFC”) is a fuel cell in which a solid electrolyte exhibiting oxide ion conductivity is used as an electrolyte. In the SOFC, since the electrolyte is solid, there is no problem of dissipation of the electrolyte, and long life can be expected. Further, as an operating temperature is as high as about 1000° C., the utility value of waste heat is high. Furthermore, as an output power density is high, the SOFC can be expected to be compact and of high efficiency.
In general, structures of this kind of SOFC are broadly divided into those of planar type, tubular type, and integral type. Among them, the planar type SOFC has advantages of high generation efficiency because internal resistance is comparatively low, and of high output power density per unit volume because thin cells are stacked.
The planar type SOFC is further broadly divided into those of a self-supporting electrolyte film type and a supported electrolyte film type. The former one ordinarily has a structure in which a plurality of single cells are stacked via a separator, wherein a fuel electrode supplied with a fuel gas such as hydrogen and city gas is bonded to one side of a planar self-supporting solid electrolyte, and an air electrode supplied with an oxidant gas such as air and oxygen is bonded to the other side of the solid electrolyte.
For the solid electrolyte used in a single cell for the SOFC, yttria-stabilized zirconia (hereinafter sometimes referred to as “YSZ”) is conventionally used. However, YSZ is of high internal resistance and shows low oxide ion conductivity. Therefore, recently, in view of improving output power density or the like of the SOFC, attention has been directed at scandia-stabilized zirconia (hereinafter sometimes referred to as “ScSZ”) with oxide ion conductivity higher than YSZ, and various researches have been conducted.
In addition, generally, for a fuel electrode material, a cermet of Ni and YSZ containing 8 mol % Y2O3 (hereinafter sometimes referred to as “Ni-8YSZ”) or the like is used. For an air electrode material, (La, Sr) MnO3 or the like is used. For a separator material, LaCrO3 or the like is used.
The present applicants have confirmed through researches that since ScSZ of a tetragonal crystal shows high oxide ion conductivity and is excellent in mechanical characteristics such as strength and toughness, the single cell using this as the solid electrolyte and the SOFC made by stacking a plurality of the single cells show excellent generating performance and reliability (see Japanese Patent Application Unexamined Publication No. 2004-055326).
However, though the above single cell and SOFC are excellent in generating performance and reliability withstanding practical use, the operating temperature is still as high as the order of about 950° C. Therefore, more improvements have been required for achieving low-temperature operation.
This is because, if operating characteristics at low temperatures can be improved without sacrificing the generating performance and reliability, great promise is shown for early commercialization.
Incidentally, in order to achieve the low-temperature operation with the above single cell and SOFC, materials suitable for the low-temperature operation should be respectively selected for the constituting members. However, depending on the combination of the materials, the generating performance and reliability could be contrarily impaired.
Further, in the single cell and SOFC, different types of constituting members are bonded to or bought in contact with each other, i.e., the solid electrolyte and the electrodes are bonded to each other and the electrodes are brought in contact with the separators. Therefore, not only material properties of the respective constituting members but also an interaction between the different types of members must be fully considered.
As mentioned above, for the single cell and SOFC, it is important to make comprehensive developments in consideration of the material properties of the respective constituting members and the interaction between the different types of members. Especially, in order to improve cell performance under disadvantageous conditions for the single cell and SOFC, i.e., the low-temperature operation, development techniques as mentioned above take on more importance.